The Geigen, whose name comes from the German geige,
meaning “violin”, is a common diapason/string hybrid.
While its tone varies between builders, it is usually (and properly)
more diapason than string. It blends well, and is often used
as the 8' foundation in Swell divisions. It is most often found
at 8' pitch, though 4' examples are not uncommon, and has also
been made at 16' pitch.

Groves dates the Geigen from around 1620 in central Germany.
It was quite popular in the 19th century, and remains popular to
this day.
Most English Geigens of the 19th century were really
Horn Diapasons.

This stop is usually made of open cylindrical metal pipes,
though it has also been made of wood; Audsley provides a
drawing, reproduced here, of a wooden Geigen by Thomas Pendlebury.
Audsley and Bonavia-Hunt insist that it never be slotted,
lest its tone become horny. It is often fitted with harmonic
bridges.
Scales cited in the literature range from 4" to 5.5" at 8' CC,
with a 1/4 to 2/7 mouth and a 1/4 to 1/3 cutup.
Adlung and Locher, however, claim it to have a “very narrow” scale.

Irwin claims that the name Violin Diapason denotes a
different stop, being stringier than the Geigen.
Skinner lists Geigen, Geigen Principal and Violin
Diapason separately, but the descriptions he gives are
compatible, though not identical.
Locher lists Violino and
Violina
as synonyms for Violin Diapason.
The names Viol Diapason, Viol Principal and Grand Viol
are mentioned only by Audsley.
There is an alternative meaning for Grand Viol.

Variants

Examples

Osiris contains 175 examples of Geigen Principal,
65 examples of Violin Diapason, 37 examples of Geigen Diapason,
31 examples of Geigen, 5 examples of Viol Principal,
3 examples each of Grand Viol and Violin Principal,
and one example of Viol Diapason.
The vast majority of these are at 8' pitch, with a handful at 16' and
4', except for Geigen Principal, where about one in six are at 4' pitch.
The oldest known examples are given below.